Customize: Bulk delete widgets

[celloexpressions]

It can be extremely frustrating to delete several widgets at once, which often happens when first setting up a theme (removing default widgets). Currently, you have to expand the widget control, then locate the remove button at the bottom, which is in a different location depending on the length of the widget control.

Once it's possible to easily recover deleted widgets (via the inactive widgets functionality), we can safely introduce an easier way to delete widgets. An obvious approach would to bring over the functionality in menus, where a bulk-deletion mode is exposed when adding new menu items. Once that's in place, the bulk-delete design can be iterated for both menus and widgets in parallel.

[westonruter]

I think this makes sense to be addressed as part of #39907.

[celloexpressions]

While a delete all button is also useful, an actual selective bulk-delete feature is also a common use case, for any time that you're removing multiple but not all widgets from an area (or at all). In fact, if themes have better defaults and widget mapping is improved when switching themes, selective bulk-deletion is likely to be used much more frequently than "delete all".

There are also usability benefits to providing better parity between widgets and menus here, particularly since there is already an established UI pattern for this feature.

[celloexpressions]

The block-based widget editor enables bulk delete through ctrl/shift-click and through techniques such as repeated backspace-ing in the editor canvas. This ticket is fixed by [50996].